1. Field of the Invention
The present invention relates to a multi-beam exposer unit, which can be used in a color printer having a plurality of drums, a color copy machine having a plurality of drums, a high-speed laser printer or a digital copy machine, for scanning a plurality of beams.
2. Description of the Related Art
In an image forming unit such as a color printer having a plurality of drums or a color copy machine having a plurality of drums, there are used a plurality of image forming sections for forming an image in accordance with color components, which are color-separated, and a laser exposer unit for providing a plurality of image data, i.e., a plurality of laser beams for every color component. As a laser exposer unit, there are known an example in which a plurality of laser exposer units are arranged and an example in which a multi-beam laser exposer unit, which is structured to be able to provide a plurality of laser beams.
In Japanese Patent Application KOKAI Publication No. 5-83485, there is an example using N sets of a light source, a cylinder lens, and glass f.theta. lens groups, and N/2 polygon mirrors wherein the number of beams is N. For example, in the case of four beams, four sets of the light sources, the cylinder lens glass f.theta. lens groups, and two polygon mirrors are used.
Moreover, for example, there is proposed an example of using a lens of a part of the f.theta. lens group in common. In other words, there is proposed a method in which two groups of f.theta. lens are set, all laser beams are passed through the f.theta. lens close to the polygon mirror, the number of f.theta. lenses far away from the polygon mirror is set to four, and each laser beam is individually passed through each lens.
Separately from the above examples, Japanese Patent Application KOKAI Publication No. 62-232344 discloses an example in which at least a part of a lens surface of the f.theta. lens is formed to have a toric surface to be used in common. Also, in Japanese Patent Application KOKAI Publication No. 62-232344, there is proposed that several f.theta. lenses are formed of plastic to improve a degree of design freedom of each lens surface and an aberration characteristic at an image-forming position. Japanese Patent Application KOKAI Publication No. 62-232344 also discloses a method in which each lens is used in common and all laser beams are passed through each lens.
Japanese Patent Application KOKAI Publication No. 5-83485 shows an apparatus in which the members corresponding to four sets of laser beam exposure units are incorporated into one housing. As compared with a case of using a plurality of laser beam exposer units, the space which is occupied by the laser exposer unit, is reduced in the case of using the multi-beam laser beam exposer unit seen in Japanese Patent Application KOKAI Publication No. 5-83485. However, from the viewpoint of the single laser exposer unit, the cost of parts and that of the assembly rise due to an increase in the number of lenses or the number of mirrors. Moreover, the size and weight as a single unit are increased. Furthermore, it is known that a curve of the main scanning line of the laser beam of each color component or deviation of the aberration characteristic on the image-forming surface such as a f.theta. characteristic becomes uneven due to deviation of the manufacture of the same f.theta. lenses, an error included in individual f.theta. lenses, or an error of the attachment.
Moreover, in the case of Japanese Patent Application KOKAI Publication No. 62-232344, since only the toric surface whose shape is not optimized is provided, there is a problem in that curve of the main scanning line is generated in any of the plurality of laser beams. There is also proposed an example in which a part of the laser beam directing to a deflector is controlled to be directed to a direction of an optical axis. However, it is difficult to sufficiently compensate for the aberration characteristic in all image-forming areas. Moreover, a refractive index of the lens formed of plastic is relatively largely changed by the change of temperature. Due to this, field curvature, curve of the main scanning line, or the f.theta. characteristic vary largely under environmental conditions over a wide range, particularly a state that the temperature condition is changed, there occurs a problem in which a field curvature, and a curve of the main scanning line, or the f.theta. characteristic vary largely. Moreover, in the case of Japanese Patent Application KOKAI Publication No. 62-232344, conditions such as achromatism on the entire area of the sub-scanning direction, curvature, distortion of the image surface, and a lateral magnification are not satisfied. Due to this, the number of lenses is increased. Furthermore, since the accuracy of the housing must be extremely highly set in order to ensure parallelism of the main scanning line of each laser beam, the manufacturing cost rises.